1. Field of the Invention
The present invention relates to a mechanical seal, and, more particularly, the present invention relates to a seal for a refueling canal of a nuclear power plant.
2. Description of the Related Art
While the present invention may be used in a variety of industries, the environment of a pressurized water reactor (PWR) nuclear power plant will be discussed herein for illustrative purposes. A typical pressurized water reactor is a cylindrical steel vessel with a hemispherical head on each end of the cylinder. The reactor holds the fuel and system components used to generate electricity then the reactor is on-line. One of the hemispherical closure heads can be removed for the purposes of refueling the reactor and performing maintenance within the reactor vessel. A peripheral flange is located near the joint between the reactor vessel and the removable closure head. This flange extends radially outward from the vessel centerline and is referred to herein as the “seal ledge.”
The reactor vessel is positioned within a shield wall forming an annulus between the seal ledge and the shield wall. The top of the seal ledge and shield wall form the base of the refueling canal, which is used to transport spent fuel from the reactor to a storage facility. At the top of the shield wall and refuel canal floor juncture there is a piece of embedded steel to which a canal floor liner is attached. The embedded steel, as part of the shield wall, forms the outer boundary of the annulus. Some nuclear plants have a wide annulus that may be two to three feet wide, while others have a smaller annulus that is as little as two to four inches wide.
Under normal operating conditions the annulus allows the reactor vessel to move in the vertical and horizontal directions, as well as allowing airflow around the reactor vessel. During refueling activities the annulus is sealed so the refueling canal can be flooded with water to reduce radiation levels. During reactor operation, however, the refueling canal is kept dry.
If air cannot flow around the vessel when the reactor is operating, then damage could occur to the nuclear instrumentation used for monitoring the core, the shield wall, and the reactor vessel supports. If the annulus cannot be fully sealed during refueling, then the refueling canal would drain down and refueling water would contact the outside of the reactor vessel. Refueling water must be cleaned from the outside surface of the vessel because such water contains boric acid, which is corrosive to the vessel base material. This vessel cleaning is costly and incurs radiation dose exposure, both of which are preventable.
One known attempt to seal the annulus to prevent leakage through the refueling canal uses a device that compresses an elastomer seal. These devices must be installed prior to refueling and then removed after refueling before the reactor operation cycle can begin. Additionally, these seals need to be inspected and replaced to ensure reliability. The inspections, installation, and removals are costly and incur plant personnel radiation exposure.
Other known attempts to seal the annulus use permanent reactor cavity annulus seals. These devices can be broken into several categories; some bear structural members on the shield wall and the seal ledge, others bear the structural members on beams extending into the annulus, while still other devices cantilever structural members over the annulus. All of these devices provide a permanent membrane that has a welded connection to the seal ledge and the shield wall.
One known attempt uses an annulus sealing device that is supported by beams that extend from the shield wall towards the reactor vessel, but do not contact the vessel. There is a U-shaped sealing feature that extends below the plane of the sealing flange. However, it is not possible to support this type of device in an annulus without support beams.
Other known attempts use annulus sealing devices that bear structural members on the seal ledge and the shield wall. However, these devices cannot be used in locations where it is not feasible to bear structural members on both the seal ledge and shield wall.
One known attempt uses a device that cantilevers over the annulus with the support structure either anchored on the shield wall or the seal ledge. The support structure forms part of the sealing membrane, and the membrane's flexibility is gained from a C-shaped flexure. However, the C-shaped flexure creates a side pocket that will be difficult to decontaminate after the refueling canal has been drained.
Thus, what is needed is a better way to seal the refueling canal.